1. Field of the Invention
The present invention relates to a control system for a two-stage actuator for a disk drive.
2. Description of the Related Art
Known as an actuator for moving a magnetic head in a magnetic disk drive is a so-called two-stage actuator having a two-stage structure such that a microactuator for slightly moving the magnetic head is mounted on a coarse actuator having a voice coil motor (VCM) as driving means. For driving of such a two-stage actuator, a control system different from a conventional one is required. That is, required is a two-stage actuator control system capable of properly controlling coarse actuator control means and microactuator control means to achieve accurate seek control and tracking control for a magnetic head.
In a conventional magnetic disk drive, an actuator having only a VCM as means for moving a magnetic head is generally used. However, there is a limit in the performance of such an actuator for moving a magnetic head at high speeds across thousands or tens of thousands of tracks in case of intending to perform positioning control (tracking control) for the magnetic head with an accuracy of tenths of a one-track width.
There has been proposed a double actuator having a coarse actuator using a VCM as driving means and a piezoelectric actuator using a piezoelectric element as driving means mounted on the coarse actuator in a magnetic disk drive. Recently, there have also been proposed a microactuator using static electricity instead of the piezoelectric element as driving means and an electromagnetic microactuator having a structure similar to the structure of the VCM.
In driving such two-stage actuators, control means different from a conventional one is required. More specifically, a transfer function of control means given by an automatic control theory must be designed. Since both the coarse actuator and the microactuator must be controlled, two control means are required. The two control means perform positioning control for one magnetic head, so that the relationship between the two control means is important.
FIG. 1 is a block diagram showing a circuit configuration of a magnetic disk drive having a two-stage actuator. A hard disk controller 1 controls an interface 3, and is provided with a cache RAM 5. A microcontroller unit 7 controls individual circuits in the disk drive. A control program is stored in a ROM 9. A read/write circuit 11 controls a magnetic head 13 to read data from a magnetic disk and write data to the magnetic disk. A spindle motor drive circuit 15 controls a spindle motor 21. A VCM drive circuit 23 controls a voice coil motor (VCM) 25. Further, a microactuator drive circuit 39 controls a microactuator 41.
As a control system for such a two-stage actuator, systems as shown in FIGS. 2 and 3 have been proposed. Referring first to FIG. 2, block 44 denotes a microactuator; block 46 denotes a microactuator controller; block 48 denotes a coarse actuator; and block 50 denotes a coarse actuator controller. Further, block 52 denotes a microactuator model using the same expression as that of the microactuator 44. In the microactuator model 52, the displacement of only the microactuator is artificially estimated.
Pm represents a transfer function of the microactuator; Pv a transfer function of the coarse actuator; Cm a transfer function of a compensator for control of the microactuator; and Cv a transfer function of a compensator for control of the coarse actuator. Further, P'm represents an equivalent filter for estimating the displacement of the microactuator; Um a drive value for the microactuator; and Uv a drive value for the coarse actuator.
The microactuator drive value Um is output from the microactuator controller 46, and the microactuator 44 is driven according to the drive value Um. The displacement of the microactuator is estimated from the output from the microactuator controller 46, i.e., from the drive value Um for the microactuator 44. As means for estimating the displacement, a filter having characteristics approximated to the characteristics of the microactuator 44 is used. The estimated displacement of the microactuator is output from the microactuator model 52, and input into the coarse actuator controller 50. The coarse actuator drive value Uv is output from the coarse actuator controller 50, and the coarse actuator 48 is driven according to the drive value Uv.
By this control, the microactuator 44 follows a target track, and the coarse actuator 48 is operated so as to maintain the displacement of the microactuator 44 always at zero. That is, an observed position y of the head is feedback controlled to coincide with a target position r. This control system is especially effective for a piezoelectric microactuator whose maximum displacement is small, and unless the servo band of a microactuator control system is made sufficiently smaller than the servo band of a coarse actuator control system, a phase margin cannot be ensured.
The open-loop characteristics (open-loop transfer function) of the control system shown in FIG. 2 are expressed as follows: EQU CmPm(1+CvPv)
In this expression, 1+CvPv has characteristics such that a gain in a low band is the same as the characteristics of CvPv and a gain in a high band is 0 dB. That is, 1+CvPv is a proportional integral regulator (PI regulator), which acts to boost only the low-band gain. The characteristics of 1+CvPv become the characteristics of PI.sup.2 or PI.sup.3.
On the other hand, CmPm represents the open-loop characteristics of the microactuator control system. In the case that the microactuator is a piezoelectric actuator, Pm may be regarded as a substantially constant gain, and Cm may be configured by a simple integrator or a double integrator.times.phase compensator. This configuration has already been published. In the case that the microactuator is an electromagnetic microactuator, a method of using a lead-lag filter has already been published in the field of an optical disk drive.
In a compensator having the characteristics of PI.sup.2 or PI.sup.3, phase lag occurs in a low band. That is, the higher the servo band of the coarse actuator control system CvPv is set, the more the phase margin is reduced. Accordingly, in this control system, the servo band of the microactuator control system CmPm and the servo band of the coarse actuator control system CvPv must be separated from each other.
Another conventional two-stage actuator control system will now be described with reference to FIG. 3. In this control system, the displacement of the microactuator 44 is estimated in the microactuator model 52, and the sum of this estimated displacement and the position error between the target position r and the observed position y is input into the coarse actuator controller 50. According to this control system, the coarse actuator follows the sum of the position error and the displacement of the microactuator. While the servo band of a control system whose sensitivity function is the product of the sensitivity function of the microactuator control system and the sensitivity function of the coarse actuator control system must be separated from the servo band of the coarse actuator control system, which of the two servo bands is higher is arbitrary.
The open-loop characteristics of this control system are expressed as follows: EQU (1+CmPm)(1+CvPv)-1=CmPm+CvPv+CmPmCvPv
This expression is identical with the expression obtained by adding CvPv to the expression of the control system shown in FIG. 2. Accordingly, the sensitivity function can be expressed as follows: EQU 1/{(1+CmPm)(1+CvPv)}
That is, the open-loop characteristics of this control system can be expressed as the product of the sensitivity function of the control system for operating the microactuator only and the sensitivity function of the control system for operating the coarse actuator only.
The open-loop characteristics substantially become CmPmCvPv in a low band and become CmPm+CvPv in a high band. Since the expression has a symmetrical form, which of the servo band of the microactuator and the servo band of the coarse actuator is higher is arbitrary. However, in the case of using a piezoelectric actuator as the microactuator, close attention must be paid to distribution of the servo bands because the maximum displacement of the piezoelectric actuator is small.
The case of using an electromagnetic actuator as the microactuator will now be considered. It is assumed that phase compensation for CmPm and CvPv is performed by a lead-lag filter. It is further assumed that the servo band of the microactuator control system and the servo band of the coarse actuator control system are equal to each other. In this case, CmPm and CvPv become asymptotic with respect to a curve of -20 dB/dec near each servo band. Accordingly, CmPmCvPv becomes asymptotic with respect to a curve of -40 dB/dec. As a result, a phase margin is reduced by the addition of CmPmCvPv. Therefore, in this control system, the servo band of the coarse actuator must be separated from the servo band of the microactuator, so as to ensure a phase margin.
In this control system, the displacement of the microactuator is estimated as similarly to the control system shown in FIG. 2. The difference between these two control systems is an input to the coarse actuator controller. That is, in the control system shown in FIG. 3, the sum of the displacement of the microactuator and the difference between the observed position and the target position of the magnetic head is input into the coarse actuator controller 50. By applying such an input to the coarse actuator controller, the control of the coarse actuator is carried out.
The control system shown in FIG. 3 has the following defects. If the servo bands (zero crossing frequency) of the coarse actuator controller and the microactuator controller approach each other, a stable control system cannot be designed. Further, the higher the gain of the coarse actuator control system is set to suppress disturbance by the coarse actuator, the less easily the phase margin of the two-stage actuator control system is ensured.
In the control system shown in FIG. 2, the servo band of the microactuator control system cannot be set lower than the servo band of the coarse actuator control system. The reason why the microactuator is intended to be used is that it is difficult for the conventional actuator using a VCM as driving means to follow variations of the target position. However, the variations of the target position have not only high frequencies such as several kHz, but also low frequencies.
For examples, in the case that the disk rotating speed is 7200 rpm, there is a large effect of disturbance having a low-frequency component including 120 Hz that is a rotational frequency and an integral multiple thereof. Even though the control is performed, it is impossible to perfectly remove such a low-frequency disturbance. Further, the displaceable distance traveled by the microactuator is limited, so that the microactuator cannot move across thousands of tracks unlike the coarse actuator using the VCM.
Accordingly, if the servo band of the coarse actuator controller is set low, the microactuator must solely follow large eccentricity. However, in some cases, this becomes difficult because of the limitation on the displaceable distance of the microactuator. It is therefore necessary to change the ratio in servo band between the microactuator controller and the coarse actuator controller according to the characteristics of the microactuator and the magnitude and frequency of the disturbance, in order to remove the disturbance more effectively.
Further, while the displacement of the microactuator is estimated from the drive value by the microactuator model in the above conventional control systems, the control is a complete open-loop control, and an actual error is not fed back. Accordingly, there is a possibility that a large difference may be produced between a position estimated by the microactuator model and an actual position of the microactuator because of errors in mechanical characteristics between the microactuator model and the actual microactuator and initial conditions at starting the control.